1. Technical Field
The present invention relates to printers that print predetermined characters or images by ejecting liquid from multiple nozzles to form fine particles thereof (dots) onto a print medium.
2. Related Art
Ink jet printers, one of such printers, print predetermined characters or images onto a print medium to produce desired prints by discharging (ejecting) liquid (ink drops) from the nozzles of a liquid ejection head (also referred to as an ink jet head) to form fine ink dots onto the print medium while relatively moving the print medium and the ink jet head. Among them, printers that move an ink jet head placed on a moving body, called a carriage, in the direction intersecting the direction of transportation of print media are generally referred to as multipass ink jet printers. On the other hand, printers capable of so-called one-pass printing with an ink jet head that is long in the direction intersecting the direction of transportation of print media (which may not be of an integral type) are generally referred to as line-head ink jet printers.
Some of these ink jet printers perform printing by applying electrical charge to, for example, a transporting belt, to charge it, transporting a substantially insulating print medium electrostatically adsorbed to the transporting belt, and ejecting ink drops from an ink jet head onto the print medium transported. Another printer transports a print medium adsorbed on a transporting belt by negative air pressure. Such print-medium transporting methods are useful particularly for line-head ink jet printers. An example of liquid-ejection timing signals (ink-drop-ejection timing signals) is a pulse signal output from a linear encoder disposed on a transporting belt, as described in JP-A-11-245383. The printer ejects ink drops in synchronism with such a pulse signal output from the linear encoder.
An ink jet printer described in JP-A-2005-75475 has two line-head ink jet heads at upstream and downstream portions of the transportation of print media and two sets of transporting units corresponding to the ink jet heads in the direction of transportation of print media, the transporting units each having a plurality of transporting belts disposed at predetermined intervals in the direction intersecting the direction of transportation of print media. This printer performs printing by transporting a print medium that is electrostatically adsorbed on the transporting belts, and ejecting ink drops onto the transported print medium from the upstream and downstream ink jet heads. The ink jet heads are disposed between the transporting belts. The troubles of the nozzles of the ink jet heads are resolved, that is, the nozzles are cleaned using a cleaning unit disposed directly under the ink jet heads.
In the case of printers having a plurality of ink jet heads along the direction of transportation of print media and a plurality of transporting units corresponding to the ink jet heads, the transporting units each having two or more transporting belts in the direction of transportation of print media, as described in JP-A-2005-75475, for correct control of liquid ejection (ink-drop discharge) timing, it is desirable that each of the transporting units have a linear encoder and a liquid-ejection timing signal (ink-drop-discharge timing signal) be output in response to the signal output from a corresponding linear encoder. However, for example, even if a linear encoder with an output signal pitch (cycle) corresponding to the print resolution is mounted on each of the transporting units disposed in direction of transportation of print media, and linear-encoder output signals are switched as a liquid-ejection timing signal (ink-drop-discharge timing signal) in accordance with the timing at which a print medium is transferred from the upstream transporting unit to the downstream transporting unit, the timing of liquid ejection (ink-drop discharge) is shifted at the position of the switching. Accordingly, the liquid cannot be ejected (dots cannot be formed) in correct positions, resulting in a decrease in print quality.